Graphene
Graphene is an allotrope of carbon, essentially a one-atom or few-layers hexagonal sheet of crystalline graphite. Graphene was first reported as isolated individual sheets in 2004. It displays remarkable properties which are suppressed in multi-layer crystalline graphite. These properties include very fast transmission of charge, the highest mechanical strength and greatest thermal conductivity yet measured, and room temperature quantum Hall effect. The quantum Hall effect is a quantum-mechanical phenomenon that is observable in very high magnetic fields at room temperature. In this condition, single atomic sheets of graphene display frictionless current flow and electrical resistances as low as a billionth of an ohm.
The available literature suggests that graphene is around 200 times stronger than steel, is more conductive than copper, can make conventional solar panels 50 to 100 times more efficient, is 50 to 100 times faster than today's semiconductors, and has the potential to make aircraft 70% lighter, charge batteries up to 10 times faster, and store up to 10 times more energy. It could also be used in the manufacture of super-capacitors for the electronics industry. Still further, graphene is likely to have applications in LEDs, touch screens, photodetectors, ultrafast lasers, membranes, spin valves and high frequency electronics and as an additive to polymers for paint coatings, electroplating solutions, composites and battery electrodes.
There are two main forms of graphene end product. These are graphene films; continuous and uniform covering the surface of a substrate, and platelets; smaller flakes (diameter <38 μm lateral dimension) that can be sold in the form of powder or in suspension.
Production of Graphene
There are many available prior art production methods for graphene, even at this early stage in graphene's development. Up to a dozen major methods are presently being used but only a small number of these are understood to be scalable. It is estimated that hundreds of tonnes of graphene are presently being produced per annum. However, no single manufacturing method stands out as the best. Each manufacturing or production method has its merits, such as quality, cost or volume. However, each method also has its drawbacks, such as highly refined or expensively prepared graphitic feedstock, or small or flawed graphene sheets or toxic by-products for example.
The source of the carbon and the type of method used to either grow graphene (bottom-up processes), or liberate graphene from a natural source of graphite (top-down processes) generally dictates the potential end use. The method employed further brings implications/limitations for the quality and level of defects, which in-turn influences the applications to which graphene is suited. Current or early applications appear most likely to use exfoliated graphene flakes or platelets (a top-down process), whereas applications that require large, defect-free graphene sheets are likely to take longer to develop (for example solid state electronic applications that require a bottom-up process).
The readily apparent flaw with all presently known methods relates to the scale of production and the prohibitive cost. There is no known method for the production of graphene that facilitates large scale bulk production of graphene for end use consumer products.
As noted above, technologies for the production of graphene may be divided into two broad categories, bottom-up or top-down, each of which include a range of specific production methodologies. All these methods require high purity graphite or highly prepared carbon materials as feedstock for graphene production processes.
Bottom-up methods mean that graphene is made (grown) using carbon molecules typically from a pure hydrocarbon source. Graphene is assembled atom by atom onto a substrate which ensures very few atomic layers and low defect films. However, the high temperature and pressure, complex equipment and handling complications required to grow and transfer the graphene makes this a very high cost approach. Specific bottom-up methods include chemical vapour deposition (CVD), growth on silicon carbide (epitaxial growth), growth on metals through precipitation, molecular beam epitaxy, and chemical synthesis using acetylene, methane or benzene as the building block.
The top-down methods mean that graphene naturally already exists and is liberated from its host, for example graphite mineral concentrate or synthetic graphite (highly ordered pyrolytic graphite—HOPG). There are many processes that rely on natural or synthetic graphite as the graphene precursor material. These methods predominantly produce flakes of graphene of variable thickness (for example, a distribution that includes a percentage of single to few layer material (1-10 atom layers) being few layered graphene (FLG) and a many layered material (10-150 atom layers) being graphene nanoplatelet (GNP)). Specific top-down processes include mechanical or micro-mechanical exfoliation/cleavage, sonication, laser ablation and photoexfoliation, anionic bonding, and electrochemical exfoliation.
A variety of techniques for the exfoliation of a variety of graphitic substances to produce graphene have been demonstrated in the prior art. However, all of these are limited to laboratory scale application and each presents obvious problems when contemplating their scaling up for the production of commercial quantities of graphene. All the current processes require a feedstock of purified natural concentrate or synthetic graphite.
Bonaccorso et al. in “Production and processing of graphene and 2d crystals”, Materials Today, December 2012, Volume 15, No. 12, pages 564 to 589; Coleman et al., ACC Chemical Research, 2013, 46(1), 14-22; and Bohm et al., patent publications WO 2015/074752A1 and WO 2015/090622A1, describe “liquid-phase exfoliation” or LPE as generally involving three steps, being the dispersion of graphite in a solvent, exfoliation and subsequent purification. These techniques typically utilise a highly processed and purified graphite source as a precursor or feedstock. Electrolytic techniques require at least one graphitic electrode for the production of graphene. These electrodes are most typically reconstituted from highly pure graphitic material. Purified starting products include synthetic graphite or natural concentrated graphite at +99% graphite (“Cg”) purity.
Graphite Mining and Processing
Typical graphite deposits of the world occur in a “hard-rock” environment, for example of 50 to 100 MPa compressive strength. Any ‘free digging’ deposit is an exception to this general graphitic rock condition.
All presently operating graphite mines are focused on the production of flake grades. That is, the production of flake particles greater than about 75 μm and at a grade of >90% Cg. Micro-crystalline flake, of so-called amorphous sizes below 75 μm, may be produced in isolation or as a low-grade by-product from larger flake production and processing.
The first step in the extraction of graphite in a hard rock deposit is mining. The objectives of mining are typically to safely provide a target feed grade to the process plant at an economic cost. The sub-processes of mining will depend upon the form/shape, topography and orientation of the mineralisation, but will typically include:                1) Grade control;        2) Drill and blast;        3) Load and haul; and        4) Blend and stockpile.        
The extraction of graphite then requires concentration of flake graphite by liberation from its host rock, a comminution process. This is followed by beneficiation processes or steps.
The initial steps in the processing of graphite ore are typically dependent upon the state of the weathering of the host rock, the flake morphology and the liberation size. For example, in highly weathered graphitic rich rock, or graphite bearing soil, the graphite mineral is generally free of the host rock whereas graphite contained within its host rock will require liberation through a comminution process.
Comminution typically comprises various stages of crushing and milling. For example, jaw crushers, rod or ball mills and cone crushers may be used to liberate the graphite and concentrate it in different size fractions, in closed circuit with classifiers. Granular contaminants (gangue) such as quartz and feldspar are ground into fines during this process whereas the graphite flake fraction is less affected and passes through to beneficiation steps. Classifying devices such as shaking screens and hydro-cyclones are typically utilised. The product of milling and screening is a slurry of particles in water that are broken down to the release size of graphite/gangue. The size distribution of particles is maximised consistent with the release of the ‘primary’ mineral (in this case graphite) so as to avoid over-breakage of flakes and smearing of graphite over gangue minerals present.
The size fractions of liberation may be checked by microscopic examination when the flake graphite has passed through initial processing. This establishes two things, first whether the correct graphite comminution size has been achieved, and secondly whether the graphite flakes have been completely freed from the host rock. If a proportion of the graphite is still attached to host rock particles, then the larger size fractions may well require further grinding. If further grinding is necessary, the next largest sieve-size graphite fraction is selected as the new liberation size and so on down the size range until a satisfactory size is achieved. Typically, sieve sizes range from 2 mm (largest diameter) down through each of 1 mm, 500 μm, 250 μm, 125 μm, and 75 μm. It is understood that graphite particles below this size tend not to concentrate effectively. Commonly, this process of regrinding and microscopic examination is repeated to the point at which all graphite particles have achieved what is considered to be an optimum liberation size.
In a concentration or beneficiation step, such as may be achieved using froth flotation techniques, contaminants must be removed, particularly any mica, quartz, feldspar, and carbonate gangue that may be present. The addition of reagents such as kerosene, dispersants, collectors and frothers, which coat the graphite flakes, while adjusting the pH of the mixture to about 7.5-8.5 through the addition of alkaline reagents to improve selectivity, is understood to help in this separation step. The objective of ‘rougher’ flotation is generation of a primary concentrate which lowers the cost of down-stream or upgrade treatment. A second or more steps of cleaner flotation further improves the sized graphite concentrates and removes any mica and other materials smeared with graphite during earlier processing.
For example, coarse particle or flake flotation is conducted mechanically agitated cells with forced air aspiration.
A combined reject or tailings slurry from flotation is pumped to disposal. Water recovery from the tails slurry may be performed via thickeners, filters or decantation from the impoundment.
Multiple stages of cleaner flotation are usually conducted on the concentrate stream. Additional dosages of reagents are often introduced at each stage. Later stages of ‘cleaner’ flotation may be conducted in various types of flotation cells, e.g. sparged columns. Gravity separation, e.g. hydraulic spirals or shaking tables, can be inserted into the flow-sheet to ‘polish’ the final concentrate.
Inter-stage regrinding of the staged concentrates, along with recycle of ‘middlings’ streams, are also common practice.
The number of stages of beneficiation, as well as the relationship between overall grade and recovery of graphite, depend on:                1) natural size distribution of the graphite particles;        2) type and texture of gangue minerals;        3) customer specifications; and        4) sophistication and experience of operating conditions.        
The beneficiated graphite concentrates are generally dried, packaged and containerised for distribution. Concentrates can be marketed across a variety of grades and sizes to suit customer requirements, including preparation or forming of graphite electrodes. Vertically integrated graphite organisations will likely further upgrade concentrates above 98% Cg to supply high value markets, for example battery electrode materials, lubricants or feedstock for graphene production.
Such treatments will depend on the residual gangue minerals and their elemental contamination:                1) micronizing and further classification or beneficiation;        2) intensive acid leaching with Hydrofluoric acid (HF) or Hydrochloric acid (HCl) or both;        3) caustic roasting and mild acid leaching.        
It should be noted that these prior art “top-down” processes are directed to the removal of gangue materials so as to produce highly purified graphitic materials and precursors for the production of graphene.
In light of the above, a process for the production of graphite/graphene that did not depend upon a purified synthetic precursor or require intensive, time consuming and expensive mining, comminution and beneficiation of natural graphite would clearly be beneficial. Further, if the expensive process of removing gangue materials and forming high purity graphitic carbon electrode material for use in the electrolytic production of graphene could be avoided, that would also be clearly beneficial. Further, if a process were developed that effectively retained the gangue material as a natural binder for a graphitic electrode, that would be beneficial. Still further, if a process were developed that allowed the production of graphene on a commercial scale that would also clearly be beneficial.
Prior art has been published that appears to suggest that a variety of graphitic material may be utilised in the electrolytic production of graphene. For example, US 20140027299/WO2013089642 describes using an “HLM rock” as an electrode in the electrolytic production of graphene. The HLM rock is described only as potentially/preferably a graphite rock, “directly mined, without any form of purification”. However, little information is provided regarding either the preferred mining method employed nor of the characteristics of the HLM rock—such as what shape or character of one rock may be suitable relative to another not being suitable. However, it can be surmised that the HLM rock is a high grade/high purity graphitic rock as little or no mention is made with regard to the impact of gangue materials that might otherwise be present. In addition, the HLM rock is clearly rock or particles of irregular or randomly broken form. Further, the electrolyte used is an “HLM-based slurry”, comprising “small pieces of HLM (eg. milled HLM)” in a mixture of organic solvent and a salt.
In US20130001089 an electrolytic method for the production of graphene is described using a first electrode that is a carbon material and a second electrode that may also be a carbon material or a metal. The solution used in electrolysis is described as any of an acid, a surfactant, a salt or an oxidising agent, or a combination of any of these. A range of potential carbon materials for the first electrode are proposed, including ‘natural graphite flakes’, highly ordered pyrolytic graphite (HOPG), carbon rod and amorphous carbon. The examples provided in this document rely on purified carbon ‘bulk’ materials, specifically natural graphite flakes, HOPG or a nano-carbon material. Whilst there is broad reference to use of a ‘carbon material’ there is no teaching present that a raw ore be utilised, nor any detail regarding what features of such a raw quarried ore may be beneficial or how traditional problems associated with using such an electrode material may be mitigated. The electrolyte is described as an anionic surfactant.
WO 2013132261 describes a process for the electrolytic production of graphene that utilises a graphitic negative electrode, a positive electrode that may be graphitic also, and a solvent that has organic and metal ions as cations. Preferred materials for the negative electrode are said to include “highly ordered pyrolytic graphite (HOPG), natural and synthetic graphite”. Further, the electrode may be “a single graphitic crystalline flake or many flakes held together”. This document goes on to suggest that in a preferred embodiment the graphitic material of the negative electrode “may be treated prior to use in order to improve its electrochemical exfoliation”. No disclosure is provided as to how a ‘natural’ graphite electrode may be provided, nor how it “may be treated”. The electrolyte used in the electrolytic exfoliation step is described as an organic solvent with both organic ions and metal ions.
US 2013/0161199 describes an allegedly large scale process for the rapid production of graphene and graphene oxide using electrochemical exfoliation. A first electrode is said to include graphite. Starting graphite material is disclosed as potentially including ‘natural graphite in a layered structure’ amongst other options, such as HOPG, coal, carbon material containing graphite flakes and so on. Further, the graphite material can be “a crystalline graphite layer material in the form of large particles, fragments, or powder”. However, no detail or disclosure is provided regarding how or why a raw graphite material might be utilised, nor as to why it might be expected to function sufficiently/effectively. Further, the electrolyte is disclosed as potentially being “hydrogen bromide, hydrochloric acid, or sulphuric acid”. An additional oxidant may be provided. Additional options of heating, use of ultrasound or microwave and/or stirring are described. To aid the provision of a continuous process, the use of a module for filtering and separating graphene products is described, for example including a microporous sieve and a filtration membrane. This final feature is described as essential to the broadest form of the invention described. The switching of bias voltages during electrolytic exfoliation is recommended/described also, potentially through a first, second and third bias.
Each of these specific prior art disclosures fails to provide any teaching as to the provision of a quarried graphitic material in a form that can be readily utilised in the electrolytic production of graphene. Any reference to ‘raw’ or ‘unprocessed’ graphite ore in the prior art is devoid of any detail regarding how that material is formed, produced or generated. Further, the prior art is typically suggestive that the mined graphitic material is of a high purity or high graphitic content. Still further, the prior art is clearly directed to laboratory scale experiments with the techniques described therein presenting clear obstacles to being scaled up to produce what may be termed industrial or commercial bulk quantities of graphene.
Micro-Nano Graphitic Material
The term “nano” is traditionally applied to a variety of materials composed of primary particles with maximum dimensions that are typically less than 0.1 micrometre (100 nm).
Material surface or structural features that are in the range of 1-10 nm are understood to be approaching “molecular size” and may have properties that are “hybrid” between the microscopic and macroscopic world. In response to the need for graphite-based nanomaterials there is a demand for graphite platelets with primary particle sizes below 100 nm. These platelets can be as small as 2.5 nm thick in the through plane (parallel with the “C” crystallographic axis) direction.
Micro graphite is a term applied to graphitic material typically ranging from 0.1-38 micron in lateral diameter, having what are believed to be excellent performance attributes in certain applications. Micro graphite is typically produced from comminution of larger “flakes” of graphite and is used in a range of industrial applications including lubricants and, in recent times, for batteries and conductive paints. Purities range from 97-99.9% Cg, however headline purity is often less significant than the types of impurities present (transition metallic impurities often diminish performance).
Conventional attrition mills can be used to reduce graphite particles to about 38 micron lateral dimension. Higher energy attrition milling can reduce particles down to about 10 micron, however cost efficiencies come into question and there are risks associated with introduction of media contaminants. So, for the most part, industry uses micronisation to reduce graphitic material to micron level sizes. The micronising process typically uses steam, jets of air and the particles themselves which collide with one another to reduce size. In so doing, the edges of the particles become altered from their natural shape to a more rounded form. Layer edges are crushed or rolled together which decreases the area and number of edges for reactions in various applications. It also results in high graphite losses where the broken edges contribute fine graphite particles that are not economic to recover.
Nano graphite is a product that is smaller and thinner again and shares characteristics closer to those displayed by graphene. It is believed that nano graphite cannot be economically produced via mechanical milling. Nano graphite generally has very high surface area material with micron/sub-micron lateral dimensions and thickness ranging from approximately 50-100 nm (150-300 atom layers). The small lateral size in functionalised form enhances mixing with hosts like polymers in a range of specialty applications.
Currently, micro-nano graphite is formed by a series of liberation and purification steps involving comminution, beneficiation, micronisation and purification, to enable the graphitic materials to be separated from gangue minerals. It has been hypothesised by the Applicant and since reported by K S Aneja et al., Graphene based anticorrosive coatings for Cr(VI) replacement, Nanoscale, 2015, 7, 17879-17888, that this process, inter alia, alters the edge morphology/topology of the graphite flakes and essentially reduces the activation state of these edges. The milling and micronisation action used in current techniques is reported (for example by Tan Xing et al., Disorder in ball-milled graphite revealed by Raman spectroscopy, Carbon 57 (2013), 515-519) to cause the flake morphology and crystallinity to become disordered or otherwise be altered as compared to the natural morphology of the starting material.
The methods and products of the present invention have as one object thereof to overcome the above mentioned problems of the prior art or to at least provide a useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge at the priority date of the application.
Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Throughout the specification and claims the term “graphene” is to be understood to include reference to monolayer graphene, “few layer graphene” (FLG) which is graphene of between one to about 10 atom layers, graphene nanoplatelets (GNP) which is about 10-150 atom layers, unless the context requires otherwise.
Throughout the specification and claims the term “mine” or “mining” is to be understood to include reference to “quarry” or “quarrying”, and other non-explosive methods of rock or ore extraction, unless the context clearly requires otherwise.
Throughout the specification and claims the term “% graphite” or “% Cg” are to be understood to refer to the same characteristic. Additionally, each term is to be understood to refer to a % (w/w).
Throughout the specification and claims reference to “micro-nano graphite” is to be understood to collectively refer to both micro and nano graphite as described hereinabove.
Throughout the specification and claims reference to “unaltered properties”, “unaltered features” or variations thereof are to be understood to referring to properties or features (including chemical, structural or physical properties and features) that remain largely or essentially as they were previous to whatever action is described as having been undertaken.
Throughout the specification and claims reference is made to the micro-nano graphite of the present invention having an edge morphology unaltered from the graphite ore from which it is produced. Further, reference is made to the effect that the micro-nano graphite of the present invention does not exhibit the folding or rounding of platelet edges. Still further, the micro-nano graphite of the present invention is said to combine the features of a high aspect ratio, natural edges and a high surface area. These references are to be understood, unless the context clearly requires otherwise, as being made in comparison to, and relative to, graphitic materials produced from prior art processes that incorporate mechanical size reduction processes, for example milling, comminution, and micronisation processes, and which invariably demonstrate damage to, mechanical working of, or modification from, the naturally occurring graphitic crystal edge structures.